Filter aids are also available, primarily for use with slurries containing very finely divided solids. The aids are solid materials which may be used to pre-coat and protect the filter and to guard the escape of occasional small particles through the filter into the filtrate. It is a requirement that such filter aids be chemically inert to the liquid component and porous to allow passage of liquid therethrough. Examples of commercially available filter aids include diatomaceous silica and expanded perlite.
Once the solid and liquid mixture has been supplied to the filter, and a major proportion of the liquid component has passed therethrough, filtration methods typically include a subsequent washing step for displacing residual liquid component from the filter cake. This usually involves supplying a washing agent to the damp filter cake to displace residual liquid component trapped in interstices between solid particles in the cake. The washing agent is usually a liquid which has a lower boiling point than the liquid component of the mixture so that it may evaporate more readily (i.e. at a lower temperature) from the damp filter cake in a subsequent drying operation.
Prior to any drying operation there is a limit to the amount of liquid component which may be removed from the damp filter cake when relying on mechanical means alone. This limit is reached when closely packed solid particles provide a barrier to prevent further passage of liquid to and through the filter cake. However closely packed the solid particles may be, there will invariably be interstices between them, filled with the liquid component. Once all available sites on the surface of the filter are occupied by solid particles, this prevents further passage of liquid through the filter and thus it is not possible to remove this residual liquid by mechanical means alone. Also, attempts to remove this residual liquid can introduce thermal drying which can concentrate the solid and as such is undesirable.
Problems associated with this entrapment of residual liquid are particularly acute for separation of mixtures containing crystalline inorganic solids, which form relatively open lattice type structures in the filter cake. Adjacent larger crystals or particles may form bridges, blocking movement of smaller particles within the cake and also retaining residual liquid in interstices between them.